The isolated strips of tortoise intestine are evaluated as a test organ for bioassay of prostaglandin E<sub>1</sub>. This preparation responded highly sensitively to PGE<sub>1</sub> and PGE<sub>2</sub> in picogram concentration range. The mean slope and the value of precision index among the doses of 0.1, 0.3 and 0.5ng/ml in final concentration were 37.7 and 0.143, respectively. And this was relatively insensitive to different prostaglandins; E<sub>1</sub>/E<sub>2</sub> ≳ 1, E<sub>1</sub>/A<sub>2</sub> ~ 50 and E<sub>1</sub> /F<sub>2<sup>α</sup></sub> ~ 100, and showed the dual responses to 5-hydroxytryptamine and histamine; initial contraction followed by relaxation. The dose-ratio inducing the relative equal contraction height for PGE<sub>1</sub>, acetylcholine, caerulein, angiotensin and barium chloride was 0.4 : 50 : 25 : 10 : 100 in this order. These results suggest that the intestinal strips of the tortoise are suitable for bioassay of prostaglandin E<sub>1</sub> and E<sub>2</sub> between the doses of 0.1 and 1.0 ng/ml level in the tissue extracts.
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